Altimeter calibration method and system based on calibration stars

Abstract

The application discloses a calibrator star-based altimeter calibration system and method, which utilizes a calibrator star carrying a calibrator to correct the system error of a spaceborne radar altimeter, thereby completing the calibration of the altimeter to the earth. The calibrator can modulate and retransmit the pulse emitted by the altimeter, and the calibration factor can be calculated according to the echo of the calibrator without relying on the ground calibration field, and then the calibration task can be completed by substituting into the ground target echo equation. The method utilizes the calibrator star to convert the calibration task from the original single "air-ground" data acquisition to an integrated calibration system of "altimeter-load platform-ground target", and the calibrator is moved from the ground to the space, so that the ground clutter can be reduced or even eliminated, the system error and the atmospheric error can be separated, and the coupling problem of the system error and the atmospheric error can be indirectly completed.

Description

Technical Field

[0001] This invention relates to the field of microwave remote sensing technology, and in particular to a calibration method and system for altimeters based on calibration satellites. Background Technology

[0002] Traditional spaceborne altimeter calibration outside of orbit mainly relies on artificial calibration fields, ground calibrators, or uniform natural calibration fields on the Earth's surface. However, there are some problems with the selection of traditional calibration fields that need to be solved.

[0003] 1. Difficulties in Site Selection for Artificial Calibration Fields: The ideal site for an artificial calibration field is flat, open terrain with a dry climate and little rainfall, good visibility, and the number of suitable artificial calibration fields both domestically and internationally is very limited. Furthermore, the calibration cycle is relatively long. For example, the Qinghai Lake radiation calibration field is limited by the size of the lake waves and the weather. When the waves are large, radiation calibration observations must be stopped. Months with more sunny days are mainly concentrated in November, December, and January of the following year. In winter, the lake surface is frozen, and the temperature is extremely cold, increasing the difficulty of calibration. The Railroad Valley radiation calibration field in the United States, ideally, should have a constant reflectance throughout the year, but it is severely affected by soil moisture. This field only has better uniformity and the best calibration efficiency in autumn.

[0004] 2. The gradual shrinking of natural calibration fields: Regions such as the Amazon rainforest and Illinois farms are areas with stable scattering coefficients. Masanobu proposed a method for obtaining satellite instrument antenna patterns based on the Amazon rainforest, utilizing distributed target imaging provided by natural calibration fields on the ground, thus improving the accuracy of antenna pattern estimation. However, according to the latest data released by the Amazon Institute for Human and Environmental Research, between August 2020 and July 2021, the area of ​​the Amazon rainforest shrank by 10,476 square kilometers, roughly equivalent to nine Rio de Janeiro cities. This undoubtedly exacerbates the already scarce natural calibration fields.

[0005] 3. Frequency limitations of natural calibration fields: Currently, for high-frequency bands of X and above, radar echoes are mainly concentrated in the vegetation canopy area, making it difficult to generate effective feedback for altimeter signals. It is difficult to achieve calibration using only a uniform natural calibration field.

[0006] 4. Difficulty in constructing atmospheric transmission attenuation models: In the process of radiometric calibration, existing models are usually used to correct atmospheric attenuation. However, in traditional calibration schemes, altimeters obtain the required backscattering coefficients by transmitting and receiving signals from ground targets. In this acquisition path, the systematic errors of the sensor itself are coupled with atmospheric attenuation errors, and it is difficult to separate the errors using ordinary data processing methods.

[0007] In summary, due to limitations imposed by the ground calibration field and atmospheric attenuation, the calibration accuracy of altimeters is difficult to improve further, and a new calibration scheme is urgently needed to solve the above problems. Summary of the Invention

[0008] In view of this, the purpose of the present invention is to provide an altimeter calibration method based on a calibration satellite. This method uses a calibration satellite to correct the systematic error of a spaceborne radar altimeter, thereby completing the altimeter's calibration to the ground.

[0009] To achieve the above objectives, the present invention provides the following technical solution:

[0010] The altimeter calibration method based on calibration stars provided by this invention includes the following steps:

[0011] The calibration satellite is positioned between the altimeter and the ground target;

[0012] When the calibration satellite passes directly below the altimeter, the altimeter transmits and receives the echo to the calibration satellite;

[0013] The echo equations of the calibration satellite and altimeter are determined based on the radar range equation;

[0014] The calibration factor can be accurately calculated using the echo from the calibration satellite, and then substituted into the signal equation of the surface target to complete the calibration task.

[0015] Furthermore, the calibration satellite is equipped with a calibrator, which can modulate and forward the pulses emitted by the altimeter.

[0016] Furthermore, the calibration factor calculated by the calibrator has high accuracy, and the calibration task can be completed by directly substituting the echo signal of the surface target without relying on the surface calibration field.

[0017] Furthermore, the altimeter receives the echo output from the calibration satellite, which is determined according to the following formula:

[0018]

[0019] Among them, DN At For the scaler's digital output, P t G is the altimeter's transmit power, G is the receive antenna gain, and G... sys λ is the receiver system gain, λ is the wavelength, and L is the receiver system gain. pt h represents the signal attenuation from the altimeter to the calibration star. t f is the distance between the altimeter and the calibration star. alt (θ A ,φ) and f trans (θ A ,φ) represent the radiation patterns of the altimeter and the active calibrator, respectively, and qσ transThis is the backscattering coefficient of the calibrator.

[0020] Furthermore, the echo results of the altimeter to the surface target are determined according to the following formula:

[0021]

[0022] Among them, DN A This indicates the digital output of the altimeter receiving ground echoes; P r For altimeter receiving power, G sys For the altimeter receiver system gain, P t Let G be the altimeter's transmit power, G be the receiver antenna gain, λ be the wavelength, and L be the wavelength. p For atmospheric attenuation, h is the distance between the altimeter and the target, and τ is the distance between the altimeter and the target. c For altimeter compression pulses;

[0023] Furthermore, the scaling factor Y L It can be represented as:

[0024]

[0025] Based on the calibration factor, the backscattering coefficient of the surface target is calculated using the following formula:

[0026]

[0027] Where σ0 represents the backscattering coefficient.

[0028] The altimeter calibration system based on a calibration star provided by the present invention includes a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the program, it implements the above-described method.

[0029] The beneficial effects of this invention are as follows:

[0030] The present invention provides an altimeter calibration method based on a calibration satellite. This method utilizes a calibration satellite equipped with a calibrator to correct system errors in a spaceborne radar altimeter, thereby completing the altimeter's ground-based calibration. The calibrator can modulate and forward the pulses emitted by the altimeter, and substitute the high-precision calibration factor calculated by the calibration satellite into the surface target echo equation to complete the calibration task.

[0031] This method utilizes calibration satellites to transform the calibration task from the original single "ground-air" data acquisition into an integrated calibration system of "altimeter-payload platform-surface target". Moving the calibrator from the ground to space can reduce or even eliminate the influence of ground clutter, greatly reducing the dependence on the ground calibration field under traditional calibration methods, separating systematic errors from atmospheric errors, and indirectly solving the problem of coupling systematic errors and atmospheric errors.

[0032] Other advantages, objectives, and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination, or may be learned from practice of the invention. The objectives and other advantages of the invention can be realized and obtained through the following description. Attached Figure Description

[0033] To make the objectives, technical solutions, and beneficial effects of this invention clearer, the following figures are provided for illustration:

[0034] Figure 1 This is a geometric schematic diagram of the calibration system.

[0035] Figure 2 This is the calibration flowchart presented by this method. Detailed Implementation

[0036] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0037] Example 1

[0038] like Figure 1 As shown, Figure 1 This is a geometric schematic diagram of the calibration system. The altimeter calibration method based on the calibration star provided in this embodiment overcomes the accuracy limitations of traditional calibration methods. The purpose of calibration is to correct the backscattering coefficient in the received echo. The echo power is expressed as follows:

[0039]

[0040] Among them, P r For altimeter receiving power, P t Let G be the altimeter's transmit power, G be the receiver antenna gain, λ be the wavelength, and L be the wavelength. p For atmospheric attenuation, h is the distance between the altimeter and the target, and τ is the distance between the altimeter and the target. c The altimeter's compressed pulse; c represents the speed of light; σ0 represents the backscattering coefficient;

[0041] The calibration of the backscattering coefficient is essentially an inquiry into the accuracy of the remaining terms in the above equation. The backscattering coefficient can be expressed as:

[0042]

[0043] The error sources in the backscattering coefficient expression are a series of systematic errors, such as received power, system gain, and atmospheric attenuation errors. In a single "altimeter-surface target" link, the systematic and atmospheric errors are coupled together. Traditional altimeter calibration methods mainly rely on surface calibration fields, which are difficult to meet the requirements of all-weather calibration, and the site selection conditions for calibration fields are quite stringent.

[0044] Based on the above problems, this method proposes a calibration method using a calibration satellite. The calibration satellite is positioned between the altimeter and the ground target. When the calibration satellite passes directly below the altimeter, the altimeter transmits and receives signals to the calibration satellite.

[0045] In this system, the altimeter transmits signals to the ground target and the calibration satellite respectively, and can receive two different echoes. It was found that there are many common terms in the echo equations of the calibration satellite, the ground target and the altimeter.

[0046]

[0047]

[0048] Meanwhile, since the calibrator (such as a transponder) on the calibration satellite modulates and relays the signal transmitted by the altimeter, the calibration factor is calculated by comparing the common terms of the two equations above:

[0049]

[0050] Between the calibration satellite deployment and the altimeter and the ground target, there is no atmospheric attenuation error in the calibration satellite echo. Under the condition that the calibration link system and atmospheric error are additive, error decoupling is achieved.

[0051] The expression for the backscattering coefficient requires solving the equations from the altimeter's digital output and the calibrator's digital output simultaneously.

[0052]

[0053] Where σ0 represents the backscattering coefficient.

[0054] The radar cross section of the calibrator itself is obtained through self-calibration, and the parameters corrected by the calibrator's self-calibration system are substituted into the correction of the backscattering coefficient parameters to complete the calibration task.

[0055] The calibrator is detached from the ground and operates on a calibration satellite that has been launched into space;

[0056] In summary, this embodiment utilizes a calibration satellite equipped with a calibrator to correct system errors in the spaceborne radar altimeter, thereby completing the altimeter's ground calibration. The calibrator can modulate and forward the pulses emitted by the altimeter, calculate the calibration factor, and directly substitute the echo signal from the ground target to complete the calibration task without relying on the ground calibration field.

[0057] The method provided in Embodiment 1 moves the calibrator from the Earth's surface to space, which can reduce or even eliminate ground clutter, separate systematic errors from atmospheric errors, and indirectly solve the problem of coupling systematic and atmospheric errors. The error separation scheme in this embodiment mainly refers to dividing the altimeter's observation of the Earth's surface target into two parts: the link between the altimeter and the calibration satellite mainly overcomes systematic errors, while the link between the altimeter and the Earth's surface includes both systematic and atmospheric errors.

[0058] like Figure 2 As shown, Figure 2 This is the calibration flowchart presented in this embodiment. The calibration error of the on-orbit radar altimeter is decomposed into systematic error and atmospheric error by using a calibration satellite equipped with a calibrator. The calibration is then completed by combining the signal equations of the surface target.

[0059] Example 2

[0060] The method will now be described in detail with reference to the accompanying drawings and implementation scheme.

[0061] This system utilizes the interaction between the altimeter and the calibration satellite and the ground target to separate and correct errors, thereby improving the altimeter calibration accuracy. The specific implementation method is as follows:

[0062] Step 1) The altimeter transmits a signal to the calibration satellite. After reflection from the calibration satellite, the radar altimeter receives the echo and outputs a digital signal.

[0063]

[0064] Among them, DN At For the scaler's digital output, P t G is the altimeter's transmit power, G is the receive antenna gain, and G... sys λ is the receiver system gain, λ is the wavelength, and L is the receiver system gain. pt h represents the signal attenuation from the altimeter to the calibration star. t f is the distance between the altimeter and the calibration star. alt (θ A ,φ) and f trans (θ A ,φ) represent the radiation patterns of the altimeter and the active calibrator, respectively, σ trans θ is the backscattering coefficient of the calibrator; A φ represents the azimuth angle; φ represents the dip angle.

[0065] Step 2) The altimeter obtains the echo signal from the surface target:

[0066]

[0067] Among them, DN A This indicates the digital output of the altimeter to the ground echo; P r For altimeter receiving power, G sys For the altimeter receiver system gain, P t Let G be the altimeter's transmit power, G be the receiver antenna gain, λ be the wavelength, and L be the wavelength. p For atmospheric attenuation, h is the distance between the altimeter and the target, and τ is the distance between the altimeter and the target. c For altimeter compression pulses;

[0068] Step 3) Then, combine (1.1) and (1.2), cancel out the common terms, and calculate the scaling factor:

[0069]

[0070] The expression for the backscattering coefficient requires solving the equations simultaneously using the altimeter's digital output and the calibrator's digital output, and then substituting the calibration factor:

[0071]

[0072] The measurement results of the calibrator on the calibration satellite are highly accurate and can correct the system parameters in the backscattering coefficient, ensuring the stability of the variables in the result of formula (1.3), thereby achieving the goal of improving the accuracy of the backscattering coefficient.

[0073] The above-described embodiments are merely preferred embodiments provided to fully illustrate the present invention, and the scope of protection of the present invention is not limited thereto. Equivalent substitutions or modifications made by those skilled in the art based on the present invention are all within the scope of protection of the present invention. The scope of protection of the present invention is defined by the claims.

Claims

1. A calibration method for an altimeter based on a calibration satellite, characterized in that: Includes the following steps: The calibration satellite is positioned between the altimeter and the ground target; When the calibration satellite passes directly below the altimeter, the altimeter transmits and receives the echo to the calibration satellite; The echo signals from the calibration satellite and altimeter are determined based on the radar range equation; The calibration factor is calculated from the echo of the calibration satellite, and then the calibration factor is substituted into the signal equation of the surface target to complete the calibration task. The calibration factor It can be represented as: ; in, For the digital output of the scaler, The distance between the altimeter and the calibration star. The signal attenuation from the altimeter to the calibration star, and These represent the radiation patterns of the altimeter and the active calibrator, respectively. The backscattering coefficient of the calibrator; Based on the calibration factor, the backscattering coefficient of the surface target is calculated using the following formula: ； in, Represents the backscattering coefficient. This indicates the digital output of the altimeter receiving ground echoes. The distance between the altimeter and the target. For atmospheric attenuation, This is a compressed pulse for the altimeter.

2. The altimeter calibration method based on a calibration star as described in claim 1, characterized in that: The calibration satellite is equipped with a calibrator, which can modulate and forward the pulses emitted by the altimeter.

3. The altimeter calibration method based on a calibration star as described in claim 1, characterized in that: The calibrator calculates a calibration factor that meets the accuracy requirements, and can directly substitute the echo signal from the surface target to complete the calibration task without relying on the surface calibration field.

4. The altimeter calibration method based on a calibration star as described in claim 1, characterized in that: The altimeter receives the echo output from the calibration satellite, which is determined according to the following formula: ； in, For the digital output of the scaler, For altimeter transmission power, For receiving antenna gain, For the altimeter receiver system gain, For wavelength, The signal attenuation from the altimeter to the calibration star, The distance between the altimeter and the calibration star. and These represent the radiation patterns of the altimeter and the active calibrator, respectively. This is the backscattering coefficient of the calibrator.

5. The altimeter calibration method based on a calibration star as described in claim 4, characterized in that: The echo results of the altimeter to the ground target are determined according to the following formula: ； in, The digital output indicating the altimeter's reception of ground surface echoes; For altimeter receiving power, For the altimeter receiver system gain, For altimeter transmission power, For receiving antenna gain, For wavelength, For atmospheric attenuation, The distance between the altimeter and the target. This is a compressed pulse for the altimeter.

6. A calibration system for an altimeter based on a calibration satellite, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that, When the processor executes the program, it implements the method described in any one of claims 1 to 5.

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